Rose calls home to let her mum know she’s okay, but Jackie abruptly bids her goodbye after claiming to have won the lottery. Believing that she heard the sound of a scuffle over the phone, Rose asks the Doctor to take her home to check. To her relief, Jackie is fine; she just left the flat to intervene when one of her neighbours, Jade, was threatened by a thug named Darren Pye. Rose knew him as a school bully, but he’s moved on to bigger and nastier things, and now he’s living on the estate with his elderly mother. Jackie also explains that she won a computer games console on a new scratchcard promotion; the cards are being given away with grocery purchases, and students dressed in giant porcupine costumes are handing out games consoles or free holiday tickets to the winners from a booth on the High Street. Since Jackie had no interest in the console, she gave it to Mickey. Rose stops in to see him as well, and he shows off the console to her and the Doctor; it plays a single game, a first-person shooter called Death to Mantodeans, in which the player’s character must penetrate an alien fortress, killing insect aliens and solving mathematical puzzles in order to open the locked doors.
The Doctor is intrigued by the puzzles, which are more complex than he’d expected, and sits down to play a game with Mickey. Rose pops out to the shops for some milk, but runs into Darren Pye. She tries not to let him provoke her into a fight, but when he starts throwing rocks at Mrs Desai, she intervenes -- and Darren pulls a knife on her. The Doctor shows up at the last moment and drives Darren off, explaining that Mickey invited him to leave when the Doctor effortlessly beat his high score. Neither the Doctor nor Mickey is aware yet that their progress was being monitored on the planet Toop, where the porcupine-like Quevvils have been eagerly waiting for a player capable of finishing the game. When the Doctor stops playing, his “carrier” is located and killed by Mantodeans, and the Quevvils’ leader, Frinel, orders his subordinates on Earth to find this player and force him to play the game to the end. The Quevvils thus teleport to Mickey’s flat and kick the door in, and he soon realises that these aren’t students in porcupine suits after all.
The Doctor comes to the same conclusion when he spots a promotional poster with a picture of a Quevvil and realises that it’s impossible for a human to fit into that shape of suit. He and Rose visit the booth where the prizes are being handed out, but are unable to get in without a winning ticket; even the Doctor’s sonic screwdriver doesn’t work, further implying that the booth’s lock is not current human technology. The Doctor and Rose buy groceries to get scratchcards, but are unable to find a winner, and Rose thus suggests dismantling Mickey’s game console to learn more about its technology. When she and the Doctor return, however, they find Mickey gone, a smell of teleportation in the air, and claw marks on the kicked-in door. The Doctor realises that the aliens came for him but got Mickey instead, and since he has no idea where to start looking for them, he decides to play another game and lure them back.
This time, the Doctor starts the game from the introductory screen, which establishes the nature of the game; presumably, the story it is telling is the truth, as nobody on Earth would believe it. The Quevvils and Mantodeans are at war, and the Mantodeans have developed a force field keyed to Quevvil biology that prevents the Quevvils from breaking into their fortress. The Quevvils have thus searched the galaxy for another species they can use as warriors, and have settled on humans. Human agents will be transported to the entrance of the Mantodean fortress with a short-range disruptor capable of shutting down Mantodean technology; they must make their way to the computer complex at the centre of the fortress, at which point the Quevvils will activate the disruptor, shutting down the force field so they can teleport inside.
This time, the Doctor does not shoot the Mantodeans; he simply evades them, racks up a high score and then stops playing, forcing the Quevvils to come fetch him. On the Doctor’s instructions, Rose hides behind the sofa and holds onto his ankle; the Quevvils have trouble craning their heads down, and thus don’t notice her crouching on the floor when they return to Mickey’s and teleport the Doctor back to their base. Rose hides as the Quevvils throw the Doctor into a cell with Mickey, and while they’re monitoring the progress of their captives’ games, Rose slips out and finds that the base is hidden in a newsagent’s cellars beneath the prize booth. Recalling from a nature programme that porcupines can’t resist salt, Rose steals a salt cellar from a chip shop and uses her last pound coin to buy 100 individual penny sweets, thus acquiring 100 scratchcards. One of the cards wins her a game console, and she uses it to enter the booth. Once inside, she pours salt over the trap door leading to the Quevvil base. The Quevvil in the booth immediately starts licking the salt off the floor, and Rose slips out of the booth and back down to the cellars, where she sees the other Quevvils climbing up into the booth to get to the salt. Once the coast is clear, Rose enters their base and frees the Doctor and Mickey.
Rather than escape straight away, the Doctor continues to play the game, as he wants to confirm a nasty suspicion. Using a map in the Quevvil control room, he and Mickey try to run their characters into each other, but find the controllers resisting, as if the Quevvils don’t want the players to catch sight of each other. The Doctor uses his sonic screwdriver to bypass the block, and when the two player characters come within sight of one another, Mickey recognises both players as local people who won a holiday on the scratchcards. The Quevvils have been kidnapping humans, fitting them with neural control discs, and transporting them to the Mantodean fortress to act as carriers for the games’ signals; whenever a game ends, that means an innocent human being has just been killed.
A Mantodean appears on screen and kills Mickey’s carrier, Mrs Hall, before he can get her to safety. The Doctor tries to walk his own carrier out of the fortress to safety, but the moment his player steps outside, his booby-trapped disruptor explodes. The Quevvils return, having finished off the salt, and the Doctor uses his sonic screwdriver to reverse the teleport controls and send Rose and Mickey back to Mickey’s flat. The Quevvils burst in and shoot at them as the Doctor activates the controls, but Rose and Mickey are successfully teleported to safety and the Doctor destroys the teleport controls to prevent the Quevvils from following. As they conduct repairs, their commander, Revik, forces the Doctor to play the game again; he refuses at first, but Revik points out that his people will just keep distributing the games and giving out more “free holidays” until the game is won. The Doctor is furious, but has little choice but to play the game again when Revik forces him to do so at gunpoint.
Rose and Mickey have been teleported back in Mickey’s flat, but Mickey has been shot in the leg. Also, his television has been stolen while they were gone. Mickey starts to go into shock, partly from the pain and partly from understanding that real human beings are dying in the aliens’ war, but as Rose calms him down, she hears movement outside and realises that whoever stole his TV is still there. The thief has gone by the time she investigates, but Mickey, slowly recovering, urges her to get out and start collecting the games before anyone else will be killed. Rose tries to ask her mother for help, only to find that Jackie has gone, leaving a note telling Rose that she’s won a holiday on the cards. Frantic, Rose tries to telephone her mum to warn her of the danger, but gets no answer.
Rose scratches off the rest of her scratchcards, but only wins another console. She tries to take the bus back to the prize booth to intercept her mum, but this time, the bus driver notices that her pass is a year out of date and kicks her off. By the time she gets back to the booth, there’s no sign of her mother. Rose uses her winning card to enter the booth and confront the Quevvil, demanding to know where her mum is, but too late it occurs to her that she’s locked in a small booth with an alien who has no qualms about killing humans. Just in time, she remembers that the booth doors open from the inside, and flees to safety. Outside, she runs into her mother’s friend Dilys, who gives Rose her own holiday ticket, claiming that she wouldn’t feel right using it after what happened to Jackie. The good news is, Jackie didn’t go on holiday; the bad news is, this is because she ran across Darren Pye with a stolen television, and, feeling that he owed Rose for hitting him earlier, he beat Jackie half to death, putting her in hospital. He also stole her purse, with Rose’s cell phone and the winning ticket inside, and since the police are now out looking for him, Rose finds herself hoping he used the winning ticket to get away.
Fortunately, when Rose caused a commotion in the prize booth, the Doctor’s guards were distracted and he was able to escape from their base. He returns to Mickey’s flat, where Mickey tells him that he’s been browsing Internet sites -- and has found someone named “alienkiller1984” selling stolen games and telling people that they can be used to kill real aliens. Rose returns home and tells the Doctor what happened to Jackie, and is upset when the Doctor chastises her for wasting time at the hospital instead of collecting more games consoles. Jackie’s home phone then rings, and when Rose answers, she hears the sound of a man running for his life and then being killed. She assumes that Darren Pye must have bumped the redial button on the stolen cell phone, and feels guilty about not feeling guilty about his death.
Rose and the Doctor set off to collect all the games consoles they can find, only to learn that Darren has been there ahead of them, using threats and extortion to steal the consoles. The Doctor has a nasty thought and pops back to Mickey’s, where Mickey reveals that alienkiller1984 is now selling winning-holiday cards for 500 quid, telling people that they can use them to dispose of people they don’t like. The Doctor is furious, but can do little without knowing the location of the Quevvils’ home planet. Mickey not entirely seriously suggests that he try to win a vacation there, and the Doctor realises that there must be a teleport key built into the winning cards. Rose produces Dilys’ winning card, and she and the Doctor return to the TARDIS, leaving Mickey to spread anti-console rumours on the Internet and try to discourage people from playing the game.
However, Darren Pye isn’t dead after all, and he now tries to force his way back into Mickey’s flat, demanding that Mickey give him the consoles that the Doctor and Rose had collected. Before he can get in, the Quevvils materialise, having repaired the transmat. Realising that Darren has witnessed their arrival, they prepare to kill him, but he reveals that he’s been helping them. Darren Pye is alienkiller1984; he was in the process of stealing Mickey’s TV when Rose and Mickey were teleported back to the flat, and when he overheard them talking about the aliens’ games, he set out to turn the situation to his advantage by stealing winning tickets and games consoles so he could sell them. Unwilling to let the Doctor and Rose interfere with his new racket, he warns the Quevvils that they’re on their way to mess up their plans. The Quevvils return to their base to warn Frinel, taking the protesting Darren with them. Mickey has overheard it all, but has no way of warning the Doctor and Rose that they’re now walking into an ambush.
The Doctor successfully locates the planet Toop by connecting the winning scratchcard to the TARDIS console, but the force field around the Mantodean fortress prevents him from travelling there to rescue the carriers. He thus materialises in the Quevvil base, in the holding cell where the holiday winners are held until more carriers are needed. One of the Quevvils’ victims is a young boy named Robert Watson, whose mother insisted that he come on holiday with her; he finds her deeply embarrassing, and often retreats into fantasies in which he is the chosen one or a boy wizard and she isn’t really his mother. Reality is much more disturbing to him: once they ended up here, she gave herself up to the aliens to protect her son, and although Robert finds his mother highly embarrassing, he’s ashamed that he did nothing to protect her. When the Doctor and Rose arrive, Robert instantly falls in love with Rose on first sight.
The Doctor and Rose try to usher the Quevvils’ victims into the TARDIS, but the Quevvils burst into the holding cell, along with Darren Pye, and capture the Doctor and Rose as well. Darren laughs as the Quevvils prepare to escort Rose away, intending to use her as the Doctor’s carrier; unable to stop them, the Doctor gives Rose a big hug and slips his sonic screwdriver into her pocket. He warns the Quevvils that he’ll reduce their planet to dust if they harm Rose, but they ignore his threats and march away four more humans to act as carriers for the game -- including Darren, who protests to no avail that he’s been helping them. The Quevvils then take the Doctor to their games room, along with another human selected at random; seeing his chance to be brave at last, Robert volunteers so that none of the others will be harmed. In the games room, the Quevvils give the Doctor a control pad and order him to send Rose to the centre of the Mantodean fortress; they need to keep the Doctor alive, but if he refuses or deviates from the proper course, then the Quevvils will kill Robert.
The Doctor is forced to pilot Rose’s body through the Mantodean fortress. Rose finds the experience both frightening and exhilarating, as the disc planted in her forehead is exploiting her muscles to their full potential, giving her superhuman speed and agility. As the Doctor walks her through the maze, he finds a headless body holding Rose’s cell phone; Darren had in fact sold the phone and Jackie’s winning ticket to this man at the pub. The Doctor has Robert pretend to panic, distracting their guard, Gerdix, while Rose picks up the phone. He then claims that there’s a problem with his control pad, and when Gerdix investigates, the Doctor seizes Gerdix’s control unit and deactivates Robert’s implant. Gerdix responds by bristling and preparing to shoot spines at them both, but Robert slaps the control disc onto Gerdix’s head, immobilising him -- and getting spines stuck in his hand. The Doctor removes the spines, but while he’s distracted, Rose is attacked by a Mantodean; fortunately, the Doctor gets back to the control pad in time and runs her to safety. He then rewires the control pad to give himself better control over Rose, despite his outrage that the Quevvils are forcing him to do this to his friend.
On the wall of the games room is a map of the Mantodean fortress, with blue and white lights indicating the position of the carriers, both active and inactive. The white lights are active games, and the blue lights are saved games waiting for the player back on Earth to resume playing. When a light goes out, that means that a carrier has been killed. The Doctor has Rose use her cell phone to call Mickey, who is surprised to hear the Doctor speaking to him in Rose’s voice; nevertheless, he follows the Doctor’s instructions and takes all the consoles he can to the youth club. Three youths -- Jason, Anil and Kevin -- have broken into the club after hours, and Mickey convinces them to help him play the game and rescue the Quevvils’ victims. Jason and Kevin search the consoles for saved games and play them according to the instructions Mickey is relaying from the Doctor, while Anil surfs the Internet, looking for other people who are currently playing games. He finds one, but the player doesn’t believe Anil’s claim that the games are dangerous and hangs up, believing that Anil is trying to distract him so he won’t win the prize at the end.
The Doctor instructs Mickey on how to remove the proximity blocker from the controllers so the carriers won’t be prevented from getting close to each other. When Mickey and his mates find a saved game, they have the player jump back and forth, and when Robert sees the movement on the map, the Doctor sends Mickey directions so he can guide the player through the maze to Rose. He then has Rose use the sonic screwdriver to deactivate the explosive booby-trap, and has Mickey march the carrier out of the fortress to safety. Mickey is frustrated to realise that human lives are now in his hands again, but again, he has no choice but to accept the responsibility. Jason and Kevin run out of saved games, and Mickey takes them to Darren Pye’s flat; there, the confused, elderly Mrs Pye lets them in, and they find four more saved games on the consoles that Darren had stolen and not yet sold. However, once they’ve run out of saved games, that still leaves some human beings trapped in the fortress, and Robert’s mother was not one of the humans who was rescued.
The Quevvils become suspicious when they see that the Doctor is no longer heading directly for the centre of the fortress, and contact Gerdix to find out what’s happening. The Doctor lulls their suspicions once by rewiring the control box and speaking through Gerdix’s implant, but Frinel eventually becomes suspicious again -- and this time, when he contacts Gerdix, the Doctor is too busy trying to save Rose from a Mantodean to respond. The Quevvils burst into the games room, find out what the Doctor has done, and march him and Robert to the main control room; there, Frinel has the other humans brought in and threatens to kill them all, one by one, unless the Doctor follows his orders. He orders another of the Quevvils to carry out their earlier threat and kill Robert, but when the Quevvil tries to burn out Robert’s mind through his implant, he finds that Robert’s implant has been removed, and thus kills another of the humans at random.
The Doctor is forced to resume the game, but as he marches Rose through the maze, they encounter Darren Pye, who is being controlled by the man Anil spoke to earlier. Believing that Anil is trying to trick him and that Rose is a rival player looking for the prize, the man prepares to shoot her. Darren has been trying to resist the implant’s control, but he doesn’t resist this impulse. However, when the Quevvils see the two carriers in close proximity, they realise that the Doctor has rewired his control box, and despite the Doctor’s protest, they reactivate the repulsion fields. Rose and Darren are forcefully catapulted away from each other, temporarily stunning them both, and as the Doctor furiously protests that the Quevvils need Rose alive and unharmed to reach the centre of the fortress, Mantodeans descend upon the stunned Darren and kill him.
The Doctor marches Rose through the final section of the maze, and Frinel leads his people into the teleport booth, intending to lead the charge into the Mantodean fortress. While they are distracted, however, the Doctor has Rose contact Mickey and tell him to make his final move. The Doctor solves the puzzle and opens the final lock, and when Rose steps into the Mantodean control room, the Quevvils activate the disruptors and teleport across to the Mantodean fortress. But while the Doctor has been keeping the Quevvils occupied, Mickey and his friends have marched the other carriers across the desert to the Quevvils’ base and right up to their control room. Thanks to their split-second timing, the disruptors shut down the Quevvils’ systems in mid-transport, after the Quevvils have been disintegrated and before they are reassembled on the other end. The Doctor removes the other humans’ control discs, teleports them back to Earth, and then destroys the teleport machinery so the few surviving Quevvils won’t ever try something like this again.
Rose finds herself surrounded by angry Mantodeans in their now dead control room, but since the Quevvils’ controls have also been shut down, she finds herself free to explain what’s been happening. The Mantodeans remove her disruptor so they can examine it and repair the damage that it’s caused, and Rose slips out of the control room before they decide to question her further. However, she now realises that she’s stuck in the middle of the Mantodean fortress without the superhuman skills that enabled her to negotiate the maze. While searching for a way out, she meets Robert’s mother, Daisy, whose game was still on pause when the Quevvils’ scheme was thwarted. The Doctor then arrives with Robert, and they are all reunited; the Mantodean force field is no longer keeping the TARDIS out, and the Doctor was able to track down Rose because his ship seems to have a soft spot for her.
As the Doctor takes Robert and Daisy back to Earth, Robert sees that Rose is shaken by her experiences, and assures her that the Doctor really does think of her as more than just a sidekick or damsel in distress. He still fantasizes about having great adventures and travelling through time and space with the Doctor and Rose, but he understands that these are just fantasies and that his place is on Earth with his mother. The Doctor and Rose see a light on in Jackie’s flat, and Rose finds that her mum has been released from hospital and is recovering from her injuries. Satisfied that all has been resolved, the Doctor decides to leave again before he is forced to admit to Mickey that he did a good job.